1. Field
The present invention relates to magnetoresistive heads. In particular, the present invention relates to setting an operating bias current for a magnetoresistive head.
2. Description of the Related Art
Magnetoresistive (MR) heads are typically employed in data storage devices, such as magnetic tape drives and disk drives, for transducing the magnetic transitions recorded on a magnetic medium into a read signal that is demodulated by a read channel. A MR head comprises an MR element having a resistance that varies in response to the magnetic field emanating from the recording medium. The read signal may be generated by applying a constant bias voltage to the MR element and measuring the change in current flowing through the MR element as the resistance varies. Alternatively, the read signal may be generated by applying a constant bias current to the MR element and measuring the change in voltage across the MR element as the resistance varies.
Increasing the bias of the MR element typically increases the quality of the read signal (increases signal-to-noise); however, setting the bias too high reduces the lifetime of the MR element. The prior art has suggested to select a nominal voltage to operate the MR element (taking into account tolerances) to ensure longevity. By estimating the resistance of the MR element, the bias current can be set to achieve the nominal voltage. Certain MR elements, such as tunneling MR elements, exhibit a negative voltage coefficient of resistance resulting in a non-linear voltage drop at higher current densities as illustrated in FIG. 1. Consequently, the error in estimating the resistance according to a linear transfer function (e.g., using two point extrapolation) induces error in computing the optimal operating bias current.
An MR element may also exhibit a negative temperature coefficient of resistance meaning that the resistance of the MR element varies inversely with ambient temperature. It therefore may be necessary to re-estimate the resistance of the MR element as the ambient temperature changes so that the operating bias current can be adjusted accordingly. If the storage device employing the MR element operates with a limited bandwidth, the speed of the algorithm for re-estimating the resistance may impact performance.
There is, therefore, a need to accurately estimate the resistance of an MR element in order to accurately estimate an optimal operating bias current. There may also be a need to quickly re-estimate the resistance of the MR element to compensate for temperature variations.